If I were to tackle a project like this I would make sure I had a decent sized hill that I had access first. Odds are your not going to have the funds or the know how to make a run in and a ramp.
If you have access to a hill, I would then completely ditch the idea of making a foam pit. It costs way to much money to do it right, and it will take you a very very long time to acquire the needed material necessary for a foam pit from scraps.
You should look into an air bag. You can get a very large airbag from china for cheaper then you might think.
If you want the ultimate setup, go with the airbag, get some of those rubber mats that they use for wrestling, and lay it ontop of the airbag. Then if you really want to go further, buy some some snowflex and attach it to the rubber mats. That would be amazing....it would allow for nice give while actually being able to land a trick without falling over from too much sag and give in the airbag. Snowflex is hella expensive I am sure you could substitute something else for it.